A big round of interviews went about the other day, and we've got more specifics about the upcoming expansion.
Some of this stuff is self-contradictory, but I think it's just a matter of different people being willing to announce different things.
The most important/exciting news is that they are both modeling the patch cycle of Legion on Mists of Pandaria's structure - having lots of patches with story development and a real progression of the expansion. As part of that, they've also said that there will be multiple post-launch raid tiers. I think we can all agree that Warlords' two-tier expansion was a failure - we're not even getting the following expansion any quicker than we would have typically.
They've also said that there will be a new dungeon in patch 7.1, but because of the new Mythic + system, this dungeon might not inherently have better gear drop out of it, as any dungeon can now have a difficulty mode where there will be relevant drops.
Other details: They're going to do a stat squish in the subsequent expansion. I know that on the Beta people have millions of health, which is pretty crazy. Ultimately, even with the stat squish, we're inevitably going to hit these big numbers now that the level cap is almost twice as much as it was originally.
Apparently the pre-patch is going to go through a couple weeks before the Legion invasion and Demon Hunters are unlocked, but that stuff will still be happening before the expansion (really hope they release that pre-patch soon!)
There's a lot more details. MMO-Champion, as usual, has a lot of summaries.